1. Field of the Invention
The present invention relates to a resonator filter for use in radio and television transmitters and receivers and other communication equipment.
2. Description of the Prior Art
As more and more signals are used in radio and television broadcasting and communications, resonator filters for selecting the frequencies of the signals to be received are required to be highly stable and reliable in their performance. There has also been a great demand for the reduction of the cost of manufacture of receivers, transmitters and other communication equipment in which the resonator filters are installed. In particular, the development of a new technology is desired for tuning circuit components in radio-frequency stages which have been difficult to improve.
Conventional resonator filters will be described with reference to the drawings. FIG. 1 of the accompanying drawings shows a basic circuit arrangement of a resonator filter. The resonator filter comprises a resonator circuit 3 composed of an inductor 1 and a capacitor 2 which are connected in parallel to each other. The resonator filter has conventionally been constructed of components as illustrated in FIGS. 2 and 3. In one prior arrangement shown in FIG. 2, an inductor component 4 and a capacitor component 5 are interconnected by conductors 6, 7. According to another prior art resonator design, a planar inductor 9 is placed on a plate-like dielectric 8, a capacitor 12 is composed of confronting electrodes 10, 11, and the inductor 9 and the capacitor 12 are interconnected by conductors 13, 14.
However, the conventional arrangements have been subjected to the following disadvantages:
In the resonator shown in FIG. 2, the inductor component 4 is large in size in comparison with the other components, and is particularly much larger in height, with the result that the equipment in which the resonator is incorporated is not rendered smaller in size and lower in profile. A ferrite core inserted in the inductor component is variable in position due to mechanical vibrations, resulting in wide drifts in tuning frequencies. The inductance of the inductor component is unstable due to a large degree of temperature-dependency of the magnetic permeability .XI. of the ferrite core, a feature which also causes tuning frequencies and also Q of the resonator filter circuit to vary widely. For keeping the tuning frequencies stable at their target settings, the components are required to be very accurately installed in predetermined positions. Where the resonator filters are mass-produced as RF resonator filters, it is difficult to maintain a desired installation accuracy and hence the tuning frequencies tend to differ greatly from their target settings and cannot be caused to converge to fixed values. Therefore, there has been difficulty experienced with the mass production of the resonator filters.
The resonator filter illustrated in FIG. 3 has suffered a problem in that the inductor and the capacitor take up a large area to thereby prevent the equipment incorporating the resonator filter from being smaller in size. The resonator filter requires at least three electrodes, that is, the inductor electrode and the confronting capacitor electrodes. Since a large quantity of electrode material having a high conductivity and hence a high cost is used, the cost of manufacture of the resonator filter is high and efforts to reduce the amount of material are impossible to make.
The resonator filter arrangements shown in FIGS. 2 and 3 further have common drawbacks. The inductor and the capacitor are constructed as separate components that are interconnected by long conductors. The long conductors tend to produce unwanted lead inductances and stray capacities which cause the resonator filters to operate is an unstable fashion and make it difficult to achieve an initial design target. Accordingly, it takes a long period of time to design resonator filters, including corrective design actions. Since the resonator filters are composed of discrete components which perform minimum functions, it has been impossible with available technological concepts to reduce the number of components used and cope with demands for an improved manufacturing process. One resultant problem is that there is a limitation on efforts to lower the cost of the resonator filters.